Several methods have been used to provide the city with water from the Collserola mountains since Roman times, when the city was known as Barcino. These methods for transporting water have evolved over time, from harnessing the force of gravity to the present-day system of pressurised piped water. The Grott tunnel is a relic of hydraulic engineering. Built in 1855 to transport water from the Vallvidrera Reservoir, it soon fell out of use. In 1908, the tunnel was reused for the first electric passenger train built in Spain. The one-car train, with room for 36 passengers, travelled through a tunnel decorated with dozens of coloured lights, and was a great success with Barcelonians keen to enjoy nature and visit the Collserola mountains. By the time this short-lived line stopped operating, it had transported close to 40,000 passengers. The tunnel was closed after the line ceased, leaving behind a mysterious iron gate that can still be seen today and which few would associate with the city’s first electric train.